25.11.3.4 The rwlock_instances Table

The rwlock_instances table lists
all the rwlock (read write
lock) instances seen by the Performance Schema while the
server executes. An rwlock is a
synchronization mechanism used in the code to enforce that
threads at a given time can have access to some common
resource following certain rules. The resource is said to be
“protected” by the rwlock. The
access is either shared (many threads can have a read lock at
the same time), exclusive (only one thread can have a write
lock at a given time), or shared-exclusive (a thread can have
a write lock while permitting inconsistent reads by other
threads). Shared-exclusive access is otherwise known as an
sxlock and optimizes concurrency and
improves scalability for read-write workloads.

Depending on how many threads are requesting a lock, and the
nature of the locks requested, access can be either granted in
shared mode, exclusive mode, shared-exclusive mode or not
granted at all, waiting for other threads to finish first.

When a thread currently has an rwlock
locked in exclusive (write) mode,
WRITE_LOCKED_BY_THREAD_ID is the
THREAD_ID of the locking thread,
otherwise it is NULL.

READ_LOCKED_BY_COUNT

When a thread currently has an rwlock
locked in shared (read) mode,
READ_LOCKED_BY_COUNT is incremented by
1. This is a counter only, so it cannot be used directly
to find which thread holds a read lock, but it can be used
to see whether there is a read contention on an
rwlock, and see how many readers are
currently active.